In all knitting machines, there are certain common requirements which are essential to produce acceptable quality knit fabric. Several of these requirements may, at first glance, appear trivial, but in the context of differing yarn properties and the non-isotropic nature of composite yarns, are particularly difficult, if not impossible, to achieve with knitting machines heretofore available.
In order for the fabric to be smooth and free from lumps, defects and unwanted variations, the knitting instrumentalities must be able to form stitches sequentially which are the same size or which vary in some predictable manner. Since producing knit fabric still remains an open loop process, prior knitting machines move each needle through a fixed predetermined path of travel while it is assumed that the stitches formed thereby will be of the proper, uniform size and shape. However, since the stitches are sequentially formed, one after another, from the same yarn, there is simply no way to alter the size of any stitch after the next stitch is formed.
In conventional knitting machines, the needles are moved longitudinally by butt engaging cams through the aforementioned predetermined path of travel. In some knitting machines, such as circular knitting machines, the needles are moved past stationary butt-engaging cams, while in other knitting machines, such as flat-bed knitting machines, the cams are moved relative to the needles. The speed and nature of such relative movement of the butt-engaging cams and the needles to effect stitch formation is severely restricted by frictional problems attendant to pushing perpendicular to the desired direction of travel and the limitations of cam design.
In this regard, the slopes of the cams must be less than 52.degree. and therefore, several needles must be moved simultaneously in order to obtain the necessary travel distances. Therefore, the yarn is fed to and captured by a succeeding needle before preceding needles have completed the stitch formation process. The capstan effect frictional problems between the yam and the knitting instrumentalities accentuate even minor variations in the yarn properties and knitting instrumentalities movements, resulting in uneven stitches and a fabric with lumps, defects and unwanted variations.
When knitting common jersey fabric, all of the stitches are the same and therefore, all of the needles are moved the same way all of the time. However, in knitting contoured knit products, the needles must be moved in different patterns from time to time. With cam systems heretofore employed, it was necessary to shunt the butts of the needles from one cam track to another by elaborate mechanical or electromechanical devices. Even with the most elaborate cam actuation system, such shunting movement is limited and the different motions that could be imparted to any given needle are severely restricted.
In designing such prior cam systems, it was assumed that the yarns to be knitted were inelastic and had a stiffness of zero. It was also assumed that these yarns were supplied from a source under uniform or constant tension. However, when the fabric is knit from covered spandex yarns, such linear assumptions yield a design that is incapable of producing uniform stitch size and a flat, smooth fabric.